Acoustic apparatus



Jan. 25, 1966 F. M. J. HUEBER ACOUSTIC APPARATUS Filed June 14, 1961 AMP AMP

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United States Patent 3,231,686 ACOUSTIC APPARATUS Fritz Max Josef Hueber, Hamburg, Germany, assignor, by mesne assignments, to Maico Electronics, Inc, Minneapolis, Minn, a corporation of Minnesota Filed June 14, 1961, Ser. No. 117,193 3 Claims. (Cl. 179-107) This invention relates to the field of acoustic apparatus and more particularly to the field of hearing aid amplifier circuits.

For persons who have experienced a hearing loss because of a damaged nerve system a problem is generally created where the threshold of hearing of the ear is quite near the threshold of discomfort for an car which has not been damaged and this threshold sometimes shifts to lower sound pressure values. The hearing of such a person therefore often has a very restricted dynamic range, that is, the range between volume or sound pressures that are fairly audible and those that cause discomfort is relatively limited or quite narrow. For problems of this type, the correction of such hearing affliction is therefore rather difiicult. Should the patient be supplied with a regular, linearly amplified hearing aid and adjust it so the threshold of discomfort of the patient can not be exceeded, then the low level sounds will be lost and if the amplification is increased so that low level sounds can be heard, then the loud sounds or noises when amplified exceed the threshold of discomfort by an appreciable amount and the patient feels a sense of ain rather than a sense of sound.

One attempt at eliminating the above mentioned problem is to make use of a peak-limiting circuit which limits the amplitudes of the previously amplified signals. An example of such a circuit would be one in which upon receiving a signal of a given amplitude, the am plification of a particular output stage would no longer operate linearly and all amplitudes going beyond a predetermined point would be cut off. This method makes use of the non-linearity characteristic which may be found in most transistor amplifier stages. This type of circuit does offer great simplicity with a relatively small number of components. However, a great disadvantage of this circuit is that it does not provide a regulation but only a limitation on the input signals and therefore cause appreciable distortion. The only advantage gained by this circuit then is that the pain level or threshold of discomfort is not exceeded by the input signals but the intelligibility of such signals is completely lost.

Another attempt at solving the problem outlined above will now be set forth. In this particular circuit, a portion of the signal voltage is taken from the final stage of the hearing aid, and after rectification and filtering, the signal is applied to one of the previous stages as a regulating voltage. In this type of circuit, a true amplification control is achieved by shifting the operating point of the input stage. That is, when a signal of large magnitude is encountered by this circuit, the regulating voltage increases and controls the operating point of the previous amplifier stage to decrease the amplification and therefore this circuit makes it possible to transmit signals of The hearing aid prior art, however, teaches that a volume control with which the patient adjusts the amount of amplification to suit his individual requirements and his surrounding environmental requirements is usually found in an intermediate stage of the amplifier. That is, in a four stage amplifier volume control will most likely be found between the second and third stages. Because of the required voltage and current in order to accomplish the regulation, the signal can only be taken from the ice output stage and this results in the volume control being contained within the regulation control loop. Therefore, an adjustment of the volume control inherently changes the operating point of the regulation circuit, that is, adjusting the volume control to lower the output volume also reduces the dynamic range of the control circuit until at a certain volume setting, no appreciable regulation occurs. In other words, the only useable regulation occurs at the highest amplification values of the amplifying circuit and it is very rarely that a person having a hearing loss will adjust the volume to this high level.

According to my invention, the disadvantage of the circuit just described can be eliminated by removing the volume control from the regulating control loop and exciting it beyond the regulating voltage control circuit. In other words, the adjustment of the volume control is substantially independent from the regulating or automatic gain control in the stage preceding it.

It is therefore a general object of the present invention to automatically regulate the amplification of a hearing aid circuit independent of the setting of the volume control.

It is still another object of the present invention to automatically regulate the gain of a hearing aid circuit so that the output signal remains within a predetermined dynamic range independent of the input signal.

It is still a further object of this invention to provide a hearing aid circuit in which the automatic gain control circuit and volume control circuit are substantially independent of each other.

It is yet another object of the present invention to provide an automatic gain control circuit driving a signal limiting device including a volume control in which the power output of the amplifier circuit is limited.

It is still a further object of the present invention to provide an automatic gain control circuit which operates substantially independent of a volume control circuit wherein the audio-frequency response may also be varied.

It is yet another object of the present invention to provide an automatic gain control circuit independent of the volume control circuit wherein the automatic gain control circuit compensates for a shift in the transistor operating point due to temperature variations.

These and other objects of the present invention will become more apparent from the accompanying drawings where similar reference characters designate corresponding parts and in which:

FIGURE 1 is a block diagram of my invention; and

FIGURE 2 is a schematic diagram of the essential part of the feedback loop.

Basically, in a hearing aid amplifier having four transistor stages, the volume control is placed between the third and fourth stages whereas the regulating voltage is obtained from the third stage ahead of the volume control to be applied to the automatic gain control. The automatic gain control circuit is created by using a special feedback amplifier which insures regulation over a wide span of amplitudes. This regulation is accomplished by rectifying the voltage obtained from the third stage of the hearing aid amplifier and amplifying the rectified voltage with a direct current amplifier. By converting the control voltage to direct current, a minimum amount of filtering is required which greatly aids in the placement of parts in a hearing aid because of the space limitation, and by resistance coupling of the feedback direct current signal, any shift in the operating point of a transistor amplifier stage due to temperature variation is automatically compensated. Generally stable operation is attempted by providing transistor amplifiers with well known temperature correction circuits to compensate for temperature influences since transistors are which is in *the form of apotentiometer. 'which appears on the wiper arm of volume control R is applied .to capacitor 18 or 19 depending upon the QPOSltlOn of the switch arm of switch 21 and the signal affected by temperature in that the collector current rises and causes the'working region or point of operation to shift. However, these circuits which are used to compensate for temperature variation generally adversely affect the amplification characteristics of the particular circuit. These shortcomings are overcome by my invention.

FIGURE 1 shows an amplifier circuit having four stages of amplification which are designated in block diagram form as V V V and V Microphone It) is connected to transistor amplifier stage V which in turn is connected to the input of amplifier stage V by a lead 11. Amplifier'stage V is connected to stage V through another connecting lead 12 and the output of amplifier stage V is connected to a rectifier circuit 6 through a connecting lead 13. Rectifier G is connected to a feedback amplifier RV through a connecting lead 14 and the output of feedback amplifier RV is connected to the input of amplifier stage V through a connecting lead 15. It will be understood that amplifiers V V "V V rectifier G, and feedback amplifier RV all have a common ground 16 to facilitate the completion of the signal circuits. A limiter designated by the reference D is connected to the output of amplifier stage V through a connecting'lead '17 and limiter D is connected in parallel with a volume control R. Volume control R is a potentiometer which has its wiper arm connected to a tone control circuit T which contains a pair of capacitors into the circuit to accomplish the desired tonal qualities Amplifier stage V desired by the hearing aid wearer. operates as an ,outputamplifier and drives'a hearing aid receiver 22 through a pair of connecting leads 23 and 24.

Limiter Dv may take on several forms but for this particular application a pair of diodes25 and 26 are connected in parallel with their poles opposing each other,

"that is they are connected in back-to-back parallel rela- 'tionship.

In operation, a signal 'is received by amplifier stage 'V and is amplified by stage V and V where the output signal from V is applied to the'rectifier G. Rectifier G .not only rectifies the signal voltage but also smooths it 'to produce a direct current voltage of predetermined no affect upon the signal which is applied to rectifier G and hence emerges as a control signal from feedback amplifier RV to amplifier stage V Should the signal appearing across the resistive element of volume control R exceed a predetermined magnitude, diodes and 26 will limit the voltage output and this occurs generally during a lag of the feedback signal 'where .there has been a relatively low sound level impressed upon microphone 10. Diodes 25 and 26 are usually selected so that it does pass all of the .signals during normal operation and thus avoids the possibledistortion.

FIGURE 2 shows the critical operation of the feedback circuit in schematic form in which microphone 10 is connected to ground 16 and to a'capacitor 27. The other side of capacitor 27 is connected to a resistance 28 which 7 has its other terminal connected to ground 16. The juncpolarity and'this voltage is applied to feedback amplifier RV. Feedback amplifier RV is a direct current type amplifier and the voltage output is applied ,to the input 'of amplifier stage V so that any large transient voltage appearing at the output of amplifier stage V3 is'immediately smoothed and theidirect current signal applied to 'the input of amplifier stage V to reduce the overall amplification. Therefore, any sudden noise orchan'ge which is detected in the sound level by microphone It) is immediately adjusted by the gain control features of feedback amplifier RV so that a signal which appears on connecting lead .17 is within a predetermined voltage range for the greater majority of sounds detected by .microphone '10. The signal which appears on connect- .ing lead 17 is thenapplied to limiter D and if the, voltage 'range just mentioned is not exceeded, limiter D has no affect and the signal is applied across volume control R The signal scription it maybe seen that any adjustment made to volume control R or tone control "T has substantially emerges from the collector of transistor T tion of capacitor'27 and resistor 28 is connected to the base of a transistor-T The emitter of transistor T is connected to ground 16 and the collector is connected to a negative voltage supply through a load resistor 30. It is of course understood that the negative voltage supply has the positive terminal connected to ground to complete the voltage circuit. A resistor 31 is also connected to the base of transistor T and has its other terminal connected to the collector of a second transistor T The collector of transistor T is connected to "the negative voltage 'supply through a load resistor 32 and the emitter of transistor T is also connected to ground 16. The base element of transistor T is connected to rectifier G through a connecting lead 33 and it will be apparent that the input to rectifier 1G conducts a signal through a connecting lead 13 such as shown in FIGURE 1. FIGURE 2 shows in particular, he automatic feedback control circuit where resistors 28 and 31 form a voltage divider for the voltage which Assuming that a strong signal appears on connecting lead 13, voltage is rectified and filtered by a rectifier G and passed on to the base .of transistor T through connecting lead 33. This voltage causes transistor T to draw current through resistor 32 and the collector potential of T shifts towards a positive value. This change of potential is applied to the base of transistor T through resistor 31 and controls the operating point of transistor T to lessen the gain .of the transistor stage. Since the volume control is not contained in this circuit, the gain control is not affected by adjustment of the volume control and thus a true regulation is achieved. Referring briefly to FIGURE 1, once the patient sets the volume control R to a comfortable level, the feedback circuit as shown in FIGURE 2 keeps the signal level automatically constant, the signal level being practically independent of the input signal level. In a particular test conducted using the invention as set out inthe schematic diagram, the output sound pressure changed currents of transistors T and T to rise slowly. But while the current is rising in transistor T the transistor T has its operating point kept at a normal value because it obtains a correction voltage through the potential applied ,across resistor 31. The amplifying ,properties of this stage are thus preserved even hile being subjected to a rising temperature. The two additional amplifying stages V and V are stabilized in the well'known manner of resist- I .ance coupling between the collector and base.

While the wiring diagram shown in FIGURE 2 couples the compensating voltage to the base of transistor T it of course wi1l-berealizedthat=the regulation voltage may also be applied to the emitter of transistor T instead of the base and achieve the same result.

From the teachings of my invention it will be seen that I have shown and described an automatic gain control circuit which is relatively isolated from the volume control while at the same time providing a high degree of stability against temperature changes in the input stages of the transistorized amplifier.

It will of course, be understood that various changes may be made in the form, details, arrangement and proportions of the parts without departing from the scope of my invention which consists of the matter shown and described herein and set forth in the appended claims.

I claim:

1. In a transistor hearing aid circuit, a circuit having automatic regulation of the output power comprising: an audio preamplifier having a plurality of stages including input and output terminals responsive to an electrical input signal to produce an output signal of larger magnitude than said input signal; a feedback control circuit including a rectifying circuit for converting alternating current signals of audio frequency to a direct current of predetermined polarity; first connecting means including said feedback control circuit for connecting said output terminal to said input terminal of said audio preamplifier to apply a controlling voltage to said input terminal which is representative of the magnitude of said preamplifier output signal; a power output amplifier having its input responsive to an electrical input signal to produce signals as its output for driving a load device; a volume control circuit having adjustable impedance elements to vary the signal output level thereof and having a pair of back-to back diodes connected in parallel manner with said volume control circuit to form a limiting device to limit said input signals received by said power output amplifier to a predetermined magnitude during a lag of signals from said feedback control circuit; and second connecting means including said volume control circuit for connecting said output terminals of said preamplifier to the input of said power output amplifier so that said volume control circuit is substantially independent of said feedback control circuit.

2. In a transistor hearing aid circuit, a circuit having automatic regulation of the output power comprising: an audio preamplifier having a plurality of stages including input and output terminals responsive to an electrical input signal to produce an output signal of larger magnitude than said input signal; a feedback control circuit including a rectifying circuit for converting alternating current signals of audio frequency to a direct current of predetermined polarity; first connecting means including said feedback control circuit for connecting said output terminal to said input terminal of said audio preamplifier to apply a controlling voltage to said input terminal which is representative of the magnitude of said preamplifier output signal; a power output amplifier having its input responsive to an electrical input signal to produce signals at its output for driving a load device; a volume control circuit having adjustable impedance elements to vary the signal output level thereof; and having means for varying the frequency response of said signal associated therewith; and second connecting means including said volume control circuit for connecting said output terminals of said preamplifier to the input of said power output amplifier so that said volume control circuit is substantially independent of said feedback control circuit.

3. In a transistor hearing aid circuit, a circuit having automatic regulation of the output power comprising: an audio preamplifier having a plurality of stages including input and output terminals responsive to an electrical input signal to produce an output signal of larger mag nitude than said input signal; a feedback control circuit including a rectifying circuit for converting alternating current signals of audio frequency to a direct current of predetermined polarity; first connecting means including said feedback control circuit for connecting said output terminal to said input terminal of said audio preamplifier to apply a controlling voltage to said input terminal which is representative of the magnitude of said preamplifier output signal and including resistance coupling between said feedback control circuit and said preamplifier input terminals so that said controlling voltage to said input terminals compensates said preamplifier for transistor operating point shift due to temperature variations; a power output amplifier having its input responsive to an electrical input signal to produce signals at its output for driving a load device; a volume control circuit having adjustable impedance elements to vary the signal output level thereof; and second connecting means including said volume control circuit for connecting said output terminals of said preamplifier to the input of said power output amplifier so that said volume control circuit is substantially independent of said feedback control circuit.

References Cited by the Examiner UNITED STATES PATENTS 2,231,867 2/1941 Allen 330-l39 2,312,260 2/1943 Miller 179-1 2,420,686 5/1947 Shaper 179107 2,802,100 8/1957 Beck et al 33029 2,824,177 2/1958 Tado 179107 2,929,998 3/ 1960 Diehl 33029 2,951,991 9/1960 Rickner et a1. 33032 2,960,661 11/1960 Bratschi 330 2,971,065 2/1961 Busse 179107 3,014,991 12/1961 Miller et al 179107 3,015,704 1/1962 Behymer 179-107 3,021,489 2/1962 Nielsen 330139 FOREIGN PATENTS 846,735 8/ 1960 Great Birtain.

ROBERT H. ROSE, Primary Examiner.

L, MILLER ANDRUS, Examiner. 

1. IN A TRANSISTOR HEARING AID CIRCUIT, A CIRCUIT HAVING AUTOMATIC REGULATION OF THE OUTPUT COMPRISING: AN AUDIO PREAMPLIFIER HAVING A PLURALITY OF STAGES INCLUDING INPUT AND OUTPUTS TERMINALS RESPONSE TO AN ELECTRICAL INPUT SIGNAL TO PRODUCE AN OUTPUT SIGNAL OF LARGER MAGNITUDE THAN SAID INPUT SIGNAL; A FFEDBACK CONTROL CIRCUIT INCLUDING A RECTIFYING CIRCUIT FOR CONVERTING ALTERNATING CURRENT SIGNALS OF AUDIO FREQUENCY TO A DIRECT CURRENT OF PREDETERMINED POLARITY; FIRST CONNECTING MEANS INCLUDING SAID FEEDBACK CONTROL CIRCUIT FOR CONNECTING SAID OUTPUT TERMINAL TO SAID INPUT TERMINAL OF SAID AUDIO PREAMPLIFIER TO APPLY A CONTROLLING VOLTAGE TO SAID INPUT TERMINAL WHICH IS REPRESENTATIVE OF THE MAGNITUDE OF SAID PREAMPLIFIER OUTPUT SIGNAL; A POWER OUTPUT AMPLIFIER HAVING ITS INPUT RESPONSIVE TO AN ELECTRICAL INPUT SIGNAL TO PRODUCE SIGNALS AS ITS OUTPUT FOR DRIVING A LOAD DEVICE; A VOLUME CONTROL CIRCUIT HAVING ADJUSTABLE IMPEDANCE ELEMENTS TO VARY THE SIGNAL OUTPUT LEVEL THEREOF AND HAVING A PAIR OF BACK-TO-BACK DIODES CONNECTED IN PARALLEL MANNER WITH SAID VOLUME CONTROL CIRCUIT TO FORM A LIMITING DEVICE TO LIMIT AND INPUT SIGNALS RECEIVED BY SAID POWER OUTPUT AMPLIFIER TO A PREDETERMINED MAGNITUDE DURING A LAG OF SIGNALS FROM SAID FEEDBACK CONTROL CIRCUIT; AND SECOND CONNECTING MEANS INCLUDING SAID VOLUME CONTROL CIRCUIT FOR CONNECTING SAID OUTOUT TERMINALS OF SAID PREAMPLIFIER TO THE INPUT OF SAID POWER OUTPUT AMPLIFIER SO THAT SAID VOLUME CONTROL CIRCUIT IS SUBSTANTIALLY INDEPENDENT OF SAID FEEDBACK CONTROL CIRCUIT. 